1. Field of the Invention
The present invention relates to surface gauging and more particularly to apparatus for measuring the contour of a scattering surface with focused electromagnetic radiation.
2. Description of the Prior Art
The manufacture of many precision articles such as elements of instrumentation and various shapes for gas turbine engines requires the close control of and the ability to determine the contour of various surfaces on such articles. The conventional gauging techniques applicable to the problem have been surveyed and for the most part noncontact optical gauging systems have been found the most appropriate to provide the relatively stringent requirements for such articles. U.S. Pat. No. 3,671,126 entitled Noncontacting Optical Probe is simply not accurate enough for some applications. One very good system which has been found successful in surface gauging is described by J. P. Waters in U.S. Pat. No. (442,269), entitled Surface Gauging By Remote Image Tracking, filed on Feb. 12, 1974 and held together with the present invention by a common assignee. The patent teaches a unique technique for remotely tracking a spot of radiation which is focused on the surface to be gauged. The technique is extremely accurate and allows contour mapping of highly irregular surfaces in a relatively short period of time. One of the shortcomings of such a system is the shadowing effect which is encountered with certain kinds of surface contours. For example, in gauging a curved surface having fin-like projections, these surface irregularities can interfere with the line of sight between the detector and the spot on the surface from which the incident radiation is scattered. Thus, various points on the surface being measured adjacent to raised or sunken areas are shadowed from the detector and no data is possible for such points with the teaching provided in the above described application.